Tungurahua

Ecuador

1.467°S, 78.442°W; summit elev. 5023 m

All times are local (unless otherwise noted)

IG reported that seismicity at Tungurahua steadily increased from 2-4 April. On 2 April two small explosions, at 0757 and 2305, were accompanied by roaring and incandescent blocks rolling down the flanks. The second explosion ejected incandescent blocks and produced an ash plume that rose 600 m. Ashfall was reported in Cotaló (8 km NW) and Chacauco (NW). Although cloud cover often prevented visual observations, an ash plume generated by an explosion at 1455 on 4 April rose 2 km above the crater and drifted SW; ash fell in Choglontus (SW). On 4 April an explosion at 1810 lasted five minutes and generated pyroclastic flows that descended the NW and N flanks. An ash plume rose 10 km above the crater and drifted SW. Another explosion at 1816 lasted four minutes and possibly generated pyroclastic flows. Tephra up to 7 cm in diameter fell in Cusúa (8 km NW) and Píllaro. Constant tremor continued, interspersed with explosions. Strombolian activity was observed during the morning of 5 April. Steam-and-gas emissions with small amounts of ash rose less than 1 km and drifted W. At 1040 an ash plume rose 2 km. On 6 April ash plumes drifted W, and Strombolian activity ejected material that was deposited 1.5 km down the flanks. Ashfall was reported on 7 April in Bilbao (W) and Cevallos (23 km NW). On 8 April steam emissions with some ash rose 200 m and drifted SW. Minor ashfall was reported in Bilbao, El Manzano (8 km SW), Juive (7 km NNW), Mocha (25 km WNW), El Manzano. Large lahars descended the Achupashal (NW) and Confesionario drainages (WSW).

Geologic Background. Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II itself collapsed about 3000 years ago and produced a large debris-avalanche deposit and a horseshoe-shaped caldera open to the west, inside which the modern glacier-capped stratovolcano (Tungurahua III) was constructed. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.